The invention relates to an optoelectronic device for detecting marks with defined contrast patterns. Such a device includes a transmitter to emit light beams, a receiver to receive light beams and an evaluation unit to which receiving signals from the receiver output are fed and in which the contrast pattern of a mark is detected with the aid of the receiving signals.
Optoelectronic devices of this type are notably embodied as barcode readers for detecting barcodes. In particular, these barcode readers are scanners for which the light beams emitted by the transmitter are periodically conducted over a scanning region with the aid of a deflection unit. The light beams reflected back by the mark to be detected, notably the barcode, have an amplitude modulation that corresponds to the contrast pattern of the mark. This amplitude modulation is evaluated in the evaluation unit for the optoelectronic device in order to decode the mark, in particular the barcode.
Extremely smooth surfaces represent a problem during the detection of marks of this type. Examples for this are barcodes used to characterize blood samples in test tubes or the like. In that case, the problem occurring is that if the transmitted light beams are oriented toward the mark so that the light beams, which are reflected back by the mark in a mirror-type or specular reflection, directly impinge on the receiver, the amplitudes of the received light beams are large enough so that the receiver is blinded and the mark consequently cannot be detected.
To counter this problem, the use of transmitters is known which operate with linear polarized light. In addition, a polarization filter consisting of a plastic film is installed in front of the receiver. For the most part, the undesirable specular reflections of the mark can be filtered out with a polarization arrangement of this type, so that essentially only diffuse light which is reflected back by the mark must be evaluated.
However, it has turned out that the filtering with polarization filters composed of plastic films is strongly dependent on the wavelengths, so that the specular reflections are filtered out in an unsatisfactory manner, depending on the wavelength range of the transmitted light beams.